The present invention relates generally to controlled current inverter systems of the type employing controlled rectifiers and utilizing an inverter circuit to supply variable voltage and frequency alternating current to a load such as an electric motor and more particularly to such systems which include means for the protection of the rectifiers of the inverter circuit from overvoltage conditions.
The controlled current inverter system is one which is now, in its basic form, becoming increasingly popular as a supply of variable voltage, current and frequency for loads such as an alternating current (a.c.) motor. In its most popular form, the system includes a source of variable direct current (d.c.) power, an inverter circuit comprised of six controlled rectifiers, for example, thyristors of the type more properly known as silicon controlled rectifiers or SCRs, and a d.c. link connecting the d.c. power source and the inverter circuit which link includes a reactor. The output of the inverter circuit is used to supply a load which, as earlier indicated, if often an a.c. motor. The output current of the system, as applied to the load, is varied by varying the output voltage of the d.c. source. The frequency is varied by controlling the controlled rectifiers of the inverter bridge. That is, by varying the rate of application of gating signals to the thyristors of the inverter circuit the output frequency is varied.
The typical controlled current inverter circuit is a symmetrical arrangement which, in the three phase embodiment, includes a bridge arrangement of three legs each consisting of a series arrangement of a controlled rectifier, a diode, a second diode, and a second controlled rectifier. The junction of the two diodes in each leg forms the output point of one phase. The three legs of the inverter circuit are connected between the positive and negative buses which connect these legs to the d.c. source. The three controlled rectifiers connected to the positive bus are normally considered as one-half of the circuit while those connected to the negative bus are considered to be the other half of the circuit. Typically, these are referred to as the positive and negative groups of the bridge. There is further included, in the customary inverting circuit, a plurality of commutating capacitors which are connected between each of the legs; that is, there is a commutating capacitor connected between each pair of controlled rectifiers in both the positive and negative groups of this circuit such that six commutating capacitors are used.
As earlier expressed, one of the most common applications of the controlled current inverter is for the control of operation of an a.c. motor, particularly an a.c. induction motor. It is well known that an a.c. induction motor represents an inductive electrical load in which the power factor varies as a function of the load on the motor. Hence, the phase angle between the motor current and motor voltage, and thus the current and voltage as seen at the output of the bridge and within the bridge, will vary. Thus, at no load the motor will appear as an almost pure inductance and the current will lag the voltage by almost 90.degree.; whereas, if the motor is loaded in either a motor operation or a regeneration operation the angle will move away from the 90.degree. point. It is further well known that semiconductor devices, especially those of the thyristor class (e.g., silicon controlled rectifiers--SCRs), are susceptable to damage by excessive voltages, even though these voltages may be of a highly transient nature. It is also well known, that the commutating capacitors utilized in controlled current inverters tend to introduce transient voltages ("spikes") which at heavy load currents can be of a fairly high magnitude. These spikes must be considered in that they add to the fundamental applied voltage. The value or magnitude of these spikes will be a function of both the load and the load current.
Past practice has been to determine the maximum possible value of the sum of peak motor voltage and the magnitude of the spikes and to use this value in the determination of the rating of the semiconductors to be used within the system. The end result of this past practice has been to require the use of semiconductor devices which have a rating far in excess of that required to properly handle the fundamental voltage which occurs across the inverter circuit. This results in a far more expensive apparatus than if only steady state handling capabilities are required.
In addition to the rating problem just mentioned, most prior art systems employed some form of protection scheme to guard against semiconductor device damage. The usual method employed was to sense the total inverter circuit current and to shut the inverter circuit down by withholding gating signals to the controlled rectifiers when the current reached some predetermined value indicative of a potential failure condition. Manual restarting was then required to place the system back into operation. This shutting down and restarting is obviously undesirable for most, if not all, applications and further led to the use of wider margins between the rated and permissible steady state capabilities of the inverter circuit.